#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define R cin>>
#define ln cout<<'\n'
#define ll long long
#define in(a) insert(a)
#define pb(a) push_back(a)
#define pd(a) printf("%.12f\n",a)
#define mem(a) memset(a,0,sizeof(a))
#define all(c) (c).begin(),(c).end()
#define iter(c) __typeof((c).begin())
#define rrep(i,n) for(ll i=(ll)(n)-1;i>=0;i--)
#define REP(i,m,n) for(ll i=(ll)(m);i<(ll)(n);i++)
#define rep(i,n) REP(i,0,n)
#define tr(it,c) for(iter(c) it=(c).begin();it!=(c).end();it++)
ll check(ll n,ll m,ll x,ll y){return x>=0&&x<n&&y>=0&&y<m;}void pr(){ln;}
template<class A,class...B>void pr(const A &a,const B&...b){cout<<a<<(sizeof...(b)?" ":"");pr(b...);}
template<class A>void PR(A a,ll n){rep(i,n)cout<<(i?" ":"")<<a[i];ln;}
const ll MAX=1e9+7,MAXL=1LL<<61,dx[8]={-1,0,1,0,-1,-1,1,1},dy[8]={0,1,0,-1,-1,1,1,-1};
typedef pair<ll,ll> P;

ll n,m;
ll u[222222],d[222222];
vector<ll> v[222222],g[222222];

ll dfs(ll x) {
  if(u[x]) return d[x];
  u[x]=1;
  if(x==n-1) {
    d[x]=1;
    return 1;
  }
  rep(i,v[x].size()) {
    ll y=v[x][i];
    d[x]|=dfs(y);
  }
  return d[x];
}

void Init() {}
void Main() {
  cin >> n >> m;
  P p[m];
  rep(i,m) {
    ll x,y;
    cin >> x >> y;
    x--,y--;
    p[i]=P(x,y);
    v[x].pb(y);
    v[y].pb(x);
  }
  if(m%2) {
    pr(-1);
    return;
  }
  dfs(0);
  rep(i,m) {
    if(d[p[i].F]&&d[p[i].S]) {
      g[p[i].F].pb(p[i].S);
      g[p[i].S].pb(p[i].F);
    }
  }
  map<P,ll> ma;
  queue<ll> que;
  ll k=m/2;
  que.push(0);
  fill(d,d+n,MAX);
  d[0]=0;
  while(!que.empty()) {
    ll x=que.front();que.pop();
    rep(i,g[x].size()) {
      ll y=g[x][i];
      if(!ma.count(P(x,y))) {
        if(k) {
          ma[P(x,y)]=ma[P(y,x)]=0;
          k--;
        } else ma[P(x,y)]=ma[P(y,x)]=1;
      }
      if(d[y]==MAX) {
        d[y]=d[x]+1;
        que.push(y);
      }
    }
  }
  rep(i,g[0].size()) {
    if(ma[P(0,g[0][i])]==1) {
      pr(-1);
      return;
    }
  }
  string s="";
  rep(i,m) s+='-';
  rep(i,m) {
    if(ma.count(p[i])) {
      if(!ma[p[i]]) s[i]='B';
      else s[i]='R';
    }
  }
  rep(i,m) {
    if(s[i]=='-') {
      if(k) {
        s[i]='B';
        k--;
      } else s[i]='R';
    }
  }
  pr(s);
}

int main(){ios::sync_with_stdio(0);cin.tie(0);Init();ll T=1;if(0)R T;while(T--)Main();return 0;}